The Right Incentive
by wendymarlowe
Summary: Detective Sergeant Lestrade helps a younger Sherlock kick his drug habit with some excellent behavioral therapy. By which I mean he fucks Sherlock senseless once a week and doesn't let Sherlock come until Sherlock solves the most interesting case to have crossed Greg's desk in the last week - but only if Sherlock's clean. (It's working.)
The tentative knock on the door was fifteen minutes overdue. Greg turned off the telly, stretched, and sauntered over to answer it. "Friday already, is it?"

Sherlock Holmes stood there in ratty black jeans, falling-apart trainers, and a grey t-shirt obviously two sizes too small but which showcased every curve and muscle in his too-skinny torso. His hair was wild and he was wearing no socks, but his expression was steady. Not currently high, then.

"Come on in." Greg stood back, waving Sherlock on into his cramped flat. Nothing fancy, not on a Detective Sergeant's salary, but he'd at least resisted the temptation to fill up the one-room space with too much clutter. He had a telly, a futon, a postage-stamp-sized kitchenette, a table, and a loo with an actual door which closed. Which was a step up from his last flat. Greg maneuvered Sherlock onto the futon, then crawled up to straddle the young man's lap.

Sherlock shifted in his seat and let his hands fall tentatively onto Greg's hips. "I'm clean," he declared.

"Hmmm." Even though Sherlock had lied about it in the past, Greg was tempted to believe him. It was obvious he was living on the streets again - there was a certain _eau de London_ in the air around him - but he didn't have the guilty look he was always so terrible at hiding. "Chemically clean, maybe, but I'm betting you're due for a shower."

Sherlock huffed. "That bad, is it?"

"Don't be like that." Greg tugged the hem of Sherlock's shirt up and over his head, easing him out of the tight fabric. _Oh yeah, definitely on the street._ Ribs more prominent than usual, stomach a little leaner. Greg silently resolved to feed Sherlock up a bit before he let him go. "I got you a present - some of that poncy shampoo and body wash you like. It's in the shower now. Go."

"But-"

"Sherlock." Greg leaned in, locked eyes with the git. "I assure you, I'll make it worth your while. For what I'm envisioning tonight, I want you to be _very clean_. Understood?"

Ooh, _that_ got through. Sherlock visibly shivered. "Yes, sir."

"Good. Off you go."

Greg waited until he heard the sound of the water running, then he quickly threw together a plate of whatever he could find in the cupboard which was both temperature-stable and easy to eat. Heaven only knew how soon Sherlock would condescend to ingest food. Most homeless kids saw the constant hunger as a reason to get off the street as soon as possible, but Sherlock didn't seem to care. "It's all transport" and all. _Bullshit_. Greg knew _exactly_ how to make Sherlock aware of his body's needs.

"I'm done." Sherlock stood in the doorway, Greg's ugly bleach-stained towel wrapped around his hips, his hair tousled and damp. He looked like a teenager again. Greg hummed in appreciation, then abandoned his sandwich-making enterprise and stalked over to pin Sherlock to the doorframe.

"I'm not gonna find any new track marks, am I?"

Sherlock shook his head, his eyes wide. "I'm clean."

"We'll see." Greg licked a broad stripe up the side of Sherlock's neck, tilting his head so he could see the pale skin by the light of the overhead fixture in the loo. Not all addicts left marks, of course, and Sherlock at twenty was already undoubtedly more knowledgeable about human anatomy than most more mature adults, but his fantastically pale skin did tend to bruise easily. Greg moved down over Sherlock's shoulder and skinny bicep, licking and tasting even as he inspected all the likely veins. Nothing in the crook of Sherlock's elbow, of course - he was too smart for that - but Greg spent a bit of extra time sucking at the sensitive skin there anyway. Somewhere above him, Sherlock gasped.

Forearm, hands, fingers, palm - all clean. And smelling faintly of five-pounds-a-bottle body wash. Good to know he'd picked correctly; Sherlock tasted delicious. Greg repeated the process on Sherlock's other arm.

"I did have a - _ooh!_ \- a fag or two," Sherlock murmured. "Yesterday. Divorced businessman made a spontaneous decision to quit and threw half a pack in the bin at the park."

"I did taste that on you," Greg said in between nips to Sherlock's sternum. "I'm trying to quit too, you bastard." He moved on to Sherlock's left nipple, eliciting a strangled moan. "Tell me about your week while I finish looking you over."

"You're not looking, you're - _mmmph_." Sherlock nearly doubled over when Greg swirled a deft tongue in his navel, but straightened back up quickly enough. "Fine. I-went-to-class-and-slept-at-the-library-last-night."

"I'm glad." Greg skirted past Sherlock's very interested dick and breathed his way down the tender line where Sherlock's thigh met his groin. "Not glad that you got kicked out of your flat again, but glad you're still going to class. I assume that's what happened?"

Sherlock _hmm_ ed. "Victor's using again. I thought it . . . better not to be around."

"Temptation?"

"He's terrible at hiding his stash."

"Good." Greg stood again and nodded toward the futon. "Go lean back and let me finish. I'm so proud of you."

Sherlock blushed and smiled awkwardly at the hint of praise. Several weeks ago, Greg had picked up on to the young man's yearning for approval. Now it was the best way to make Sherlock docile in a hurry. Greg followed behind him, waiting until Sherlock was partially reclined against the futon's back before dropping to his knees at the edge and popping Sherlock's big toe into his mouth.

Yeah, this was why they worked so well together. Why he kept letting a kid ten years his junior come around once a week, making him feel like every single one of those premature grey hairs was deserved. Because Sherlock made the most delicious sounds when Greg sucked his toes, licked at the backs of his knees, got him hot and wanting without once touching his cock. Sherlock had his head back, eyes closed, mouth hanging open, and he moaning with absolutely no shame whatsoever.

There were more veins, Greg knew, more potential injection sites, but Sherlock was half out of it already and Greg wasn't a patient man. He finished with one last lick to the arch of Sherlock's foot and stood. Sherlock sat up straighter, eyes locked on Greg's groin. Eager for what they both knew would happen next.

"You ready to listen?" Greg stripped off his shirt in one smooth motion. He didn't miss Sherlock's hard swallow. "Get me ready, then. Mouth full, ears open, and stay _silent_ while I'm talking. No touching yourself and no trying to actually get me off before we're ready."

Almost the moment he was done speaking, Sherlock's long fingers were scrabbling at Greg's fies and tugging his pants and jeans down to his thighs. Sherlock practically gulped down Greg's cock and let out a muffled sigh of relief.

"My beautiful boy." Greg allowed himself a moment to just caress Sherlock's damp curls, trying to focus his mind on his most recent case instead of on the bloody gorgeous way Sherlock's body was twisting as he fought to fit every inch of Greg's cock in his mouth without using his hands. "Right. So this week's best case was a tricky one. I think you're going to like it. Two sisters, twins, killed the same way in the same room but three days apart. First one was a hairdresser in Romford, second was a forensic accountant with Barclays in central London. Both blonde, pretty in a rather plain way, twenty-eight. Both unmarried but with long-term boyfriends."

Sherlock pulled off far enough to look up at him, eyes bright and mouth still stretched around the head of Greg's cock. Perfectly silent, but eloquent nonetheless. _Fuck_.

"The first sister," Greg continued in a bit of a shaky voice, "was visiting her aunt and uncle out in the country outside Eton. Nice big house - nothing palatial, but sunny and open. Good design. Except for her guest bedroom; that was smaller and without windows. Just a twin bed, a desk on the opposite wall, and an old wardrobe. Totally undecorated, and very much out of place compared to the rest of the house. Aunt found her the next morning in her bed. Apparently just up and died in her sleep."

Sherlock snaked a slender arm under Greg's cock to play with his bollocks while he sucked. Greg had to stop for a few deep breaths before continuing.

"Second sister, the accountant, came two days later to answer questions about her sister and to start making preparations for the funeral. Stayed overnight in the same room. Same bed, although she wasn't told that. The house has several guest rooms - no reason to guess her sister had been in what had originally been the housekeeper's quarters instead of one of the nicer ones. Second sister was found dead the next morning. Exact same way. Aunt screamed the house down, or so I hear. We got the tail end of the case because of the suspicious circumstances and because the local police were backed up and over budget already. And because the sisters both grew up in London. Okay, that's enough." Greg pulled out of Sherlock's warm mouth - absolute hell to give it up - but Sherlock was already grabbing the lube out of the end table drawer before Greg even got his jeans and pants the rest of the way off. Greg waved it away. "My turn now. Same rules as usual - you've got twenty yes-or-no questions, although you've never needed more than half of those. And you can't come until you solve the case."

Sherlock flipped over with impressive grace and curled himself around the seat of the futon, his knees curled underneath the seat and his lovely arse on perfect display. Greg knelt between Sherlock's ankles and leaned forward to deliver a gentle bite to one cheek.

"Right." Sherlock already sounded a little hoarse and a lot breathless, which was doing nearly as much for Greg's rock-hard erection as the blow job had. "Poisoned, obviously. Tox screen results positive?"

"Nothing on the initial screening, but you're right that it seemed obvious. We had our techs run a broader panel. Both sisters came back positive for bungarotoxins."

"The krait," Sherlock responded immediately. "Twelve sub-species, spreading from Iran to Indonesia. One of the deadliest snakes in south-east Asia. Interesting."

"We thought so too." Greg palmed the twin globes of Sherlock's arse, running the tip of his nose up and down his crack. Sherlock had obviously taken the "very clean" directive to heart. "A follow-up autopsy examination discovered two faint puncture marks on both sisters' necks. Odd angle, but consistent with a snakebite."

Sherlock was silent for a minute. Normally Greg would assume he was lost in thought, but apparently Sherlock was _very_ much a fan of rimming. Nearly as much as Greg was, it seemed. Whatever Sherlock had been intending to say, it probably dissipated about the time Greg's tongue traced its way down to Sherlock's hole and Greg settled in to tease. Sherlock was practically trembling with the effort to keep his hips still, but he couldn't keep from whining a bit when Greg dipped a bit inside.

"Still waiting for your second question," Greg murmured against Sherlock's pale skin.

"I, ah." Sherlock bit back a moan. "Was there an air vent above the bed?"

Greg laughed out loud at that, startling a sharp noise from Sherlock. _Bloody psychic_. "There was, as a matter of fact. Series of vents just below the ceiling, connecting the various servants' quarters together. Ease of heating and cooling, presumably, although they weren't very efficient. The one in this room just connected to the unused room on the other side of the wall."

"Mmm." Sherlock squirmed as Greg executed another good lick. "No one else in the house?"

"The sisters' aunt and uncle. No staff except a gardener who comes on the weekends and a cleaning lady who shows up four hours a week. Neither of them overnight." He groped unerringly for the lube currently lying on the cushion next to Sherlock's twitching torso, not pulling his head back far enough to see what he was doing. "It's a big house, though, so can't rule out an intruder. They kept the patio unlocked."

"Sir, _please_." Sherlock thrust his arse out and groaned. "Too hard. Can't think."

"Too bad." Greg gave the underside of Sherlock's bollocks a few gentle brushes with the back of his index finger, then sat back on his heels and smeared some lube onto his other hand. "Next question."

"Were the sisters rich?"

"Not particularly." Greg traced Sherlock's already wet hole, then eased a fingertip in. Sherlock gasped. "Their grandmother - the aunt's mother - is, but she's in failing health. Both the aunt and the sisters' father are still alive, though, so the sisters weren't set to inherit anytime soon."

"Ngh."

"Fifth question?"

"The - _oh!_ \- the room. Was there-" Sherlock squirmed backward onto Greg's fingertip and pressed his face into the futon cushion "-was there anything odd about the room? Bed bolted in place?"

"How the fuck did you guess?" Greg rewarded him with a few slow thrust-and-retreats, then lubed up a second finger. "Yes, it was. As was the desk. Bed against the wall under the ventilation grate, and the desk on the opposite wall. Desk chair and the wardrobe were the only mobile furniture in the room."

"Solved it," Sherlock announced, popping his head back up and twisting to look at Greg over his left shoulder. "The uncle poisoned both of them with a homemade double-syringe-on-a-stick mechanism, lowered in through the ventilation grate."

Greg added a second finger. "Not a poisonous Indian snake?"

"Poisonous kills you if you ingest it; venomous kills you to ingest _you_. Totally different words." Sherlock actually _rolled his eyes_. While being fingered. Greg's quick jab to Sherlock's prostate cured him of the put-on attitude rather quickly and drew a rather undignified gasp from the younger man. "Wanted-it-to-look-like-snakebite," Sherlock rattled out all at once. _"Please."_

"Motive first." Greg couldn't resist another nibble on Sherlock's delicious arse, scraping his teeth in counterpoint with the motion of his fingers. "Come on, my genius boy. Do this and then I'll fuck you."

 _"Ooh."_ Sherlock's top half went boneless against the futon, but his lower half stayed perfectly poised to receive Greg's attentions. "Uncle stealing from grandmother. Forensic accountant sister would find out after grandmother died. May have already been suspicious. Killing sister was a ruse to get accountant sister in the house, using her laptop at a desk where her uncle could look through the duct behind her and watch her input her password. Hence desk bolted to the floor. Bungarotoxins are paralytics; small puncture point and relatively quiet death. Question uncle, find laptop, and _dear god please fuck me now."_

 _Damn_. Greg figured he'd never get used to how this scrawny, perpetually-homeless kid could pull together information like that. "Took my team four days to come to the same conclusion. And yes, the uncle had been draining his mother-in-law's accounts. I'm so proud of you, Sherlock. Would you like to come?"

 _"Yes,"_ Sherlock breathed. "Yes, sir. Tell me I've earned it."

"Oh, you have." Greg removed his fingers and replaced them with his cock, the slit just barely breaching Sherlock's slick entrance. "You're so smart when you're sober, Sherlock. Beautiful and brilliant. And I really, truly am proud of you for staying clean another week. How long has it been, now?"

Sherlock shivered. "Two months, sir."

"Two months of me buggering you boneless when you earn it." Greg ripped the foil off the condom he'd been keeping in his pocket and rolled it on in record time. "You love that, don't you?"

"God, yes." Sherlock clenched his hole, making it pulse against the tip of Greg's cock, and Greg almost gave in right there. Instead he slathered another palmful of lube onto himself and slowly, _slowly_ slid forward. They moaned in tandem.

"So warm," Greg murmured. "Warm and tight and perfect. You don't want to lose this, do you?"

Sherlock shook his head no.

"Don't want to give up, shoot up, and go the rest of your life without ever feeling my cock in your arse again?"

"Need more," Sherlock panted.

And Greg gave it to him. Amidst a long string of _brilliants, amazings, gorgeouses_ , and other nonsense praise, Greg set up a relentless rhythm which - judging from the variety and volume of noises Sherlock was making - felt just as good bottoming as it did topping. He kept his hands tight on Sherlock's bony hips, holding Sherlock's pelvis still and trying not to squeeze to tight he bruised him. Sherlock was clearly far beyond caring.

As so often happened when the two of them did this, Sherlock came first. Greg could sense the tightening in Sherlock's back muscles, could hear the sharp note to his desperate little whimpers. He wrapped his own slick hand around Sherlock's cock just in time for Sherlock to stiffen his entire body in one sharp jerk and then absolutely come his fucking brains out. Greg thrust deeper, reveling in the feel of Sherlock spasming around him, until the sensation pushed him over the edge and he emptied himself into the condom with a loud grunt.

"Christ." Greg twisted and shoved at Sherlock - who always went near-comatose after sex - until he was lying lengthwise on the futon and Greg could squish himself in alongside. "I really am proud of you, you know," he whispered.

"Mmmm." Sherlock smiled groggily.

"You got a place to stay tonight? If I put the futon flat it can sleep two."

"I'll be fine."

He didn't sound particularly convincing, but Greg didn't push. "Eat something when you're ready, then. And borrow a pair of my socks. You'll ruin your feet like that."

"Hmph. You'd be sad about that, wouldn't you?"

Greg got his face within three inches of Sherlock's, letting him see just how sad a development that would be. "Don't even try to pretend you don't enjoy it."

Sherlock blinked once, twice, then melted even more completely into the cushion. And against Greg's body. "You've got the chocolate biscuits I like?"

"And a ham sandwich, a glass of orange juice, and some of those little mustard pretzels. I know you by now."

"Mmm." Sherlock let his eyes slide closed. "You do. I like that."

Greg lay there and let himself doze. It was never safe to let yourself depend on an addict, but Friday sex with Sherlock had rapidly become the best part of his week. _Maybe someday I'll make DCI and he'll be long enough clean to help with the really big ones._ The physical relationship wasn't going to last forever - one evening a week with Sherlock was about all Greg figured he could handle - but it was hard not to hope for a long and rewarding friendship.

Beside him, Sherlock slept.


End file.
